1. Technical Field
The present disclosure relates to a transmission device and a reception device for conducting communication using a multi-antenna.
2. Description of the Related Art
Conventionally, for example, MIMO (Multiple-Input Multiple-Output) is well known as a communication method using a multi-antenna. In multi-antenna communication typified by MIMO, pieces of transmission data of a plurality of series are modulated, and each modulated signal is transmitted from a different antenna simultaneously to increase the transmission speed of data.
FIG. 1 illustrates a configuration example of a transmission and reception device having two transmit antennas, two receive antennas, and two transmission modulated signals (two transmission streams). In the transmission device, encoded data is interleaved, the interleaved data is modulated, and frequency conversion and the like is performed to generate transmission signals, and the transmission signals are transmitted from antennas. A scheme simultaneously transmitting different modulated signals from different transmit antennas at an identical frequency is a spatial multiplexing MIMO scheme.
At this point, PTL 1 proposes a transmission device provided with a different interleave pattern for each transmit antenna. That is, the transmission device in FIG. 1 is provided with two different interleave patterns having two interleaves (πa and πb) different from each other. In the reception device, as described in NPLs 1 and 2, reception quality is improved by iteratively performing a detection method (a MIMO detector in FIG. 1) in which a soft value is used.
A models for an actual propagation environment in wireless communication includes an NLOS (non-line of sight) environment typified by a Rayleigh fading environment and an LOS (line of sight) environment typified by a Rician fading environment. The transmission device transmits a single modulated signal, and the reception device performs a maximal ratio combining on the signals received by a plurality of antennas and demodulates and decodes the signals obtained by the maximal ratio combining. Therefore, the excellent reception quality can be achieved in the LOS environment, particularly in the environment having a large Rician factor that indicates a ratio of received power of a direct wave to received power of a scattered wave. However, depending on a transmission scheme (for example, a spatial multiplexing MIMO system), there occurs a problem that the reception quality degrades when the Rician factor increases. (see NPL 3)
FIG. 2 illustrates an example of a simulation result of a BER (Bit Error Rate) characteristic (a vertical axis indicates BER while a horizontal axis indicates a SNR (Signal-to-Noise power Ratio)) when data encoded by LDPC (Low-Density Parity-Check) codes is transmitted through a 2×2 (two transmit antennas and two receive antennas) spatial multiplexing MIMO system in the Rayleigh fading environment and the Rician fading environment with the Rician factors K of 3, 10, and 16 dB. FIG. 2A illustrates the BER characteristic of Max-log-APP (A Posteriori Probability) without performing the iterative detection (see NPLs 1 and 2), and FIG. 2B illustrates the BER characteristic of Max-log-APP with the iterative detection (five iterations) (see NPLs 1 and 2). As is clear from FIG. 2, regardless of the iterative detection, the reception quality degrades in the spatial multiplexing MIMO system when the Rician factor increases. Thus, it is clear that the problem in that “the reception quality degrades when the propagation environment is stabilized in the spatial multiplexing MIMO system”, which does not exist in the conventional single modulation signal transmission system, is generated in the spatial multiplexing MIMO system.
Broadcasting or multicast communication is service necessary to adapt to various propagation environments because a broadcasting station or a base station simultaneously transmits information to many terminals, and the LOS environment exists obviously in the radio propagation environment between a receiver owned by a user and the broadcasting station. When the spatial multiplexing MIMO system is used in the broadcasting or multicast communication, possibly the receiver generates a phenomenon in which the service can hardly be received due to the degradation of the reception quality although received field strength is high. That is, when the spatial multiplexing MIMO system is used in the broadcasting or multicast communication, there is a demand for development of the MIMO system in which a certain degree of reception quality is obtained in both the NLOS environment and the LOS environment.
NPL 4 describes a method for selecting a codebook (a precoding matrix (also referred to as a precoding weight matrix)) used in precoding from feedback information transmitted from a communication partner. However, NPL 4 does not disclose a method for performing the precoding in a situation in which the feedback information can hardly be acquired from the communication partner like the broadcasting or multicast communication.
On the other hand, NPL 5 discloses a method for switching the precoding matrix over time. The method can be applied even if no feedback information is available. NPL 5 discloses that a unitary matrix is used as the matrix used in the precoding and that the unitary matrix is switched at random. However, NPL 5 does not disclose a method applicable to the degradation of the reception quality in the LOS environment, but NPL 5 describes the simply random switching. NPL 5 describes neither a precoding method for improving the degradation of the reception quality in the LOS environment, nor a method for structuring the precoding matrix.
PTL 2 discloses a specific method for changing the precoding matrix in the case that two streams are subjected to the precoding to transmit the modulated signals from two antennas.